For purposes of this specification and claims, an "unplumbed sterilizer" is herein defined as a self-contained sterilizer having a) an internal liquid reservoir, b) a sterilizing chamber which is charged or filled with a preset volume of liquid, for example, water, from the reservoir to which the sterilizing chamber is fluidly connected, and c) a heater operatively connected with the sterilizing chamber which converts the liquid in the sterilizing chamber into a sterilizing fluid, for example, saturated steam. Since the sterilizing fluid is generated within the sterilizer, a sterilizer is not "plumbed" or connected to an external source of steam or other sterilizing fluid. Thus, unplumbed sterilizers are self contained and generally used as bench-top units in laboratories or the like. While the description herein will often refer to the liquid in the reservoir as water and the sterilizing fluid as steam, other liquids and sterilizing fluids may be used.
Many unplumbed sterilizers utilize a mica-type heater as the heating element. The heating element is held in place on the bottom of the sterilizing chamber by numerous bands, for example, two to six bands, extending around the perimeter of the sterilizing chamber. The ends of the bands are fastened or screwed together to hold the heater tightly in place. With such a heater, a thin sheet of mica is disposed between the heating element and the bottom of the sterilizing chamber. Heat is transferred from the heating element to the mica principally by radiation, and heat is most efficiently transferred from the mica to the sterilizing chamber by conduction. Thus, preferably, the mica must maintain a continuous and intimate contact with the bottom of the sterilizing chamber for optimum heat transfer. However, as the temperature of the chamber increases, the mica experiences a distortion; and at some locations, the mica will lose contact with the surface of the sterilizing chamber. Consequently, there is generally a nonuniform distribution and rate of transfer of heat between the heater and the bottom surface of the sterilizing chamber. Different areas or spots will transfer heat at different rates resulting in different temperatures, a nonuniform heating of the water. As the pool of water in the chamber shrinks, the nonuniform, or spot heating, is more apparent and can result in excess temperatures and superheated steam.
The nonuniform heat transfer to the chamber results in, on average, a less than desired rate of heat transfer from the heater into the chamber. Therefore, the heater must operate for longer periods of time. Typically, the temperature of the sterilizing chamber during the steam generation cycle is monitored by a heat sensor on the chamber, and the temperature is controlled by varying the power to the heater. In situations where the heat transfer is relatively poor, in order to maintain the desired temperature within the chamber, the heating element may be powered to levels exceeding its specified operating parameters. Either of the above conditions often shorten the life of heating element.
More recently, the mica-type heaters have been replaced by a heater assembly in which the electrical heating element is embedded in an aluminum casting that is shaped to conform to the bottom surface of the sterilizing chamber. The cast heater is held in place by a plurality of bands that wrap around the sterilizing chamber, the ends of which are fastened together to hold the heater in place. The cast aluminum heater has better heat transfer properties than the mica heater and, therefore, provides a more uniform heat transfer from the heater into the sterilizing chamber. However, with increased temperatures, the cast aluminum heater experiences a substantial expansion. That expansion, in conjunction with the rigid straps holding the heater in place, produces distortions and deflections which result in nonuniform contact between the heater casting and the bottom surface of the sterilizing chamber. Any such discontinuities of contact result in a less efficient rate of heat transfer from the heater into the chamber.
Thus, from the above, known heaters exhibit several deficiencies. First, during heating cycles, the expansion and contraction of the heater unit and the bottom surface of the sterilizer produce areas in which there is limited or no contact between the heater surface and the bottom surface of the sterilizing chamber. Further, such inconsistencies in surface contact area result in less than optimal rates of heat transfer from the heater into the chamber. In the steam generating cycle, such areas may produce hot spots within the chamber leading to unsaturated superheated steam. In the drying mode after the sterilizing process is complete, the relatively nonuniform and inconsistent heat transfer of known heaters may result in widely varying process times for the drying cycle. Further, such inefficiencies not only extend process cycle times, but require the heating element to be operated longer and at higher temperatures, thereby reducing cycle efficiency and heater life. Consequently, there is a need to provide an improved heater for an unplumbed sterilizer, which is more efficient and has a longer life.
A further disadvantage of prior heaters is that the multiplicity of band clamps are located around the sterilizing chamber between the sterilizing chamber and the outer body of the sterilizer. Thus, in the event of a heater element failure, to replace the heating element, the sterilizer must be substantially disassembled and then reassembled in addition to removing and reapplying the plurality of band clamps. Consequently, there is a need to provide a heater for an unplumbed sterilizer that is more easily maintained and replaced.
With prior systems, the temperature of the sterilizing chamber is monitored with a thermocouple or a resistance temperature device attached to the sterilizing chamber, and a sterilizer control modifies or changes the sterilizing chamber temperature by increasing or reducing electrical power to the heating element. With such systems, there is no control over whether power increases to the heating element are within the heating element specifications. Consequently, there is a need to better control the operation of the heater to extend its operating life.